In recent years, various sizes of optical elements have been manufactured and put into market, for example, from large size optical elements utilized as such as a telescope lens and a microscope lens to small size optical elements, for example, pickup lenses of such as CD (Compact Disc) and DVD (Digital Video Disc). Specifically, development of an optical pickup lens (an objective lens) applicable to at least one of BD (Blue-ray Disc), CD and DVD has been actively carried out in accordance with a proposal of a standard on BD or AOD (Advanced Optical Disc). On these optical elements, a functional layer depending on applications is generally formed to obtain a specific property determined by a combination of the function of the layer and a wavelength range of light irradiated to the functional layer.
For example, in Patent Document 1, disclosed is an optical element on which an antireflection layer is formed, and the antireflection layer works on a transmitting light of an ultraviolet region. On this optical element, a film of a high refractive index material (12) prepared from neodymium fluoride is formed directly on a synthetic quartz substrate (11) and a film of a low refractive index material (13) prepared from aluminum fluoride is formed directly on the high refractive index film. In Patent Document 1, it has been proved that the optical element exhibited a sufficient antireflective function (refer to paragraph Nos. 0016-0019, FIGS. 1 and 2). Herein, the above high refractive index material refers to a material having a refractive index of 1.7 or more, and the low refractive index material refers to a material having a refractive index of 1.55 or less.
Since ultraviolet light has a short wavelength and large energy of light, a technique to provide a film exhibiting a different refractive index on the surface of a lens body, as disclosed in Patent Document 1, is useful to obtain an antireflective function. However, an objective lens having a high refractive index antireflection film on a lens body made of a resin tends to have a problem in that the lens surface facing to an optical information recording medium is deformed due to a local heat generated at the surface and subsequently conveyed to the lens body, the local heat being generated when the antireflection film absorbs the energy of light at the surface facing to the optical information recording medium side surface. Further, when the generated local heat is conveyed to the lens body, due to the difference in thermal expansion coefficients of the antireflection film and the lens body, the abrasion resistance of the antireflection film may be reduced. However, when the antireflection film is not provided on the lens body, the transmittance of light decreases and the practicality of the objective lens as an optical element may be reduced.
(Patent Document 1) Japanese Patent Publication Open to Public Inspection (hereafter referred to as JP-A) No. 8-179103.